Theresa May has called Donald Trump to raise Britain’s “deep concern” over plans to impose tariffs of 25% on imports of steel and 10% on aluminium amid US threats of a trade war with China and escalating tensions with the EU.

The prime minister had been scheduled to call the US president on Sunday to discuss the appalling situation in Syria, with the pair agreeing it was a humanitarian catastrophe driven by the Syrian regime and its backer, Russia, according to Downing Street.

But May also turned to the question of Trump’s comments on trade.

Trump aides defend tariffs move as Europe and China decry 'trade war'

Read more

“The prime minister raised our deep concern at the president’s forthcoming announcement on steel and aluminium tariffs, noting that multilateral action was the only way to resolve the problem of global overcapacity in all parties’ interests,” said a No 10 spokeswoman.

Trump’s protectionism has been communicated through Twitter, where he claimed his country was losing billions of dollars on trade, adding that “trade wars are good, and easy to win”.

He also threatened taxes on European cars after the EU responded to the decisions on steel and aluminium by saying it could target US imports such as Harley-Davidson motorbikes, Levi’s jeans and Kentucky bourbon whiskey.

Donald J. Trump (@realDonaldTrump)

If the E.U. wants to further increase their already massive tariffs and barriers on U.S. companies doing business there, we will simply apply a Tax on their Cars which freely pour into the U.S. They make it impossible for our cars (and more) to sell there. Big trade imbalance!

May has strongly signalled a desire to negotiate a bilateral trade agreement with the US after Brexit, but critics said Trump’s “America first” rhetoric underlined the potential danger of being too reliant on a transatlantic relationship.

Senior government figures told the Guardian that plans to keep Britain closely tied to European standards after Brexit, which were outlined by the prime minister on Friday, had also cut the chances of a major trade deal with the US.

One cabinet minister said sticking to EU regulations in a number of areas, particularly around industrial goods, would constrain the breadth of any future UK-US trade links.

Sam Lowe, a trade expert at the Centre for European Reform thinktank, agreed, arguing that convergence in those areas makes “doing a deal with the US incredibly difficult as the majority of their aggressive objectives involve removing regulatory barriers to trade, especially when it comes to food standards”.

He added that it would make sense for the UK to keep its focus on European links. “I’d question the logic of running into a trade deal with a president who sees trade less as a means of achieving mutual prosperity and more an instrument of war.”

However, Jacob Rees-Mogg, the Tory MP who chairs the influential European Research Group, argued that the UK could still do a deal with the US, accepting American standards for imports into Britain. He said that could work as long as any goods falling below European standards were not then re-exported to the EU.

May’s speech, in which she reiterated her red lines around the single market and customs union but promised close cooperation beyond that, has led to a fragile unity on the government’s backbenches.

There has been some angst at cabinet level about briefings that have emerged in recent weeks. Cabinet ministers pushing for a softer Brexit were said to be irritated after an awayday at Chequers to agree the contents of the speech was immediately characterised by sources as “divergence has won the day”.

“It was more like divergence as a last resort,” said another source with knowledge of the meeting.

Senior MPs who supported remain, such as the Treasury select committee chair, Nicky Morgan, made clear they were happy with May’s promise to prevent customs checks on the Irish border, believing that would secure a sensible deal overall.

May rebuffed the suggestion that Boris Johnson, the foreign secretary, had raised the spectre of a hard border between Northern Ireland and the Republic in an internal government report handed to the prime minister.

“No. Boris is absolutely clear that there won’t be a hard border,” she told the BBC1’s Andrew Marr Show.

Asked about the controversial paper in which Johnson said it would be “wrong to see the task as maintaining ‘no border’ on the island of Ireland” and that even with a hard border 95% of goods could pass without checks, May challenged the interpretation of his comments.

“He’s clear that there won’t be a hard border between Northern Ireland and Ireland, and we’re working to that,” she said. “We’ve got proposals as to how we can achieve that, now we’re going to be able to sit down and talk with others about how we do that.”

Six former foreign secretaries on Brexit, Britain… and Boris

Read more

Her comments came after claims that her Gavin Barwell, her chief of staff, had leaked Johnson’s paper to Sky News. Whitehall sources said there were suspicions that the leak had originated from Downing Street or the Treasury, despite a strong denial from Barwell. They also said tMay had blocked Johnson’s plea to publish the full document to place the argument in context.

The prime minister reiterated her hopes for a customs agreement that would prevent a hard border. But Simon Coveney, Ireland’s deputy prime minister, told Marr he was unsure whether the EU27 nations would be able to support the plan and that they would be worried about protecting the integrity of the single market.

“While of course we will explore and look at all of the proposed British solutions, they are essentially a starting point in negotiations as opposed to an end point,” Coveney said.